Wind power is considered one of the cleanest, most environmentally friendly energy sources presently available, and wind turbines have gained increased attention in this regard. A modern wind turbine typically includes a tower, a generator, a gearbox, a nacelle, and one or more rotor blades. The rotor blades capture kinetic energy of wind using known airfoil principles. For example, the wind turbine may include one or more pitch drive systems configured to pitch the rotor blades into the wind. As such, the rotor blades transmit the kinetic energy in the form of rotational energy so as to turn a shaft coupling the rotor blades to a gearbox, or if a gearbox is not used, directly to the generator. The gearbox steps up the inherently low rotational speed of the rotor for the generator to efficiently convert the rotational mechanical energy to electrical energy, which is fed into a utility grid via at least one electrical connection. A power converter is typically used to convert a frequency of a generated electric power to a frequency substantially similar to a utility grid frequency. Conventional wind turbines also typically include a main controller to control various operational modes of the wind turbine.
The pitch drive system typically includes a motor that rotates the blades to a desired pitch angle to adjust an amount of wind energy captured by the blades. Known pitch drive systems use a direct current (DC) motor having one of the following possible electrical connections between a stator and a rotor of the DC motor: series, shunt, or compound. A series-wound DC motor includes an armature and field windings connected in series with a common DC power source. In addition, series-wound DC motors are characterized by having very high starting torque, but generally poor speed regulation. In contrast, a shunt DC motor includes an armature and field windings connected in parallel with a common DC power source. As such, shunt-wound DC motors are generally characterized by having very good speed regulation, but do not have the starting torque of series-wound DC motors. The compound DC motor includes an armature and field windings having both shunt-wound and series-wound characteristics, thereby having benefits of both a shunt-wound and a series-wound DC motor. Compound DC motors, however, are typically more complex and expensive than series-wound or shunt-wound DC motors.
Accordingly, a pitch drive system having a DC motor that addresses the aforementioned problems would be welcomed in the technology. For example, a series-wound DC motor that incorporates a current-controlling device between a battery assembly and a series-field winding would be advantageous.